This invention relates to a fuel injector for use in the delivery of fuel to a combustion space of an internal combustion engine. In particular, the invention relates to a fuel injector of the type intended for use in a fuel system of the accumulator or common rail type, the injector being of the type controlled using a piezoelectric actuator.
In a known piezoelectrically actuated fuel injector, a piezoelectric actuator is operable to control the position occupied by a control piston member, the piston being moveable to control the fuel pressure within a control chamber defined, in part, by a surface associated with the valve needle of the injector to control movement of the injector. The piezoelectric actuator includes a stack of piezoelectric elements, the energisation level, and hence the axial length, of the stack being controlled by applying a voltage across the stack. The lower end of the piezoelectric stack carries an anvil member including a part-spherical recess which engages a part-spherical upper region of the control piston member in a sealing manner. A small volume is defined between these two components such that, in use, when the axial length of the piezoelectric stack is reduced causing the anvil member to move in an upwards direction, fuel pressure within the volume is reduced, serving to draw the control piston member to move with the stack.
A disadvantage of this type of arrangement is that high quality surfaces are required on the anvil member and the control piston member to achieve a level of sealing which is sufficient to ensure the control piston member is caused to move with the anvil member. Furthermore, the seal formed between these components is sensitive to debris which may become trapped between them. These disadvantageous effects can cause the anvil member and the control piston member to become separated, in use, during injecting stages of the fuel injection cycle.
It is an object of the invention to provide a fuel injector in which the disadvantageous effects described hereinbefore are reduced.
According to the present invention, there is provided a piezoelectrically actuable fuel injector comprising an accumulator volume within which a piezoelectric stack is arranged, the accumulator volume being arranged to receive fuel from a source of pressurized fuel, in use, the piezoelectric stack carrying an end member which engages a surface associated with a piston member so as to apply a retracting force to the piston member upon the axial length of the piezoelectric stack being reduced, the piston member being operable to control fuel pressure within a control chamber, a volume being defined between the end member and the surface associated with the piston member, the volume being provided with vent means to permit fuel within the volume to flow to a low pressure drain.
As the volume defined between the end member and the surface associated with the control piston member is able to vent to low pressure, the end member and the surface are more firmly engaged due to fuel under high pressure within the accumulator volume. Thus, upon retraction of the axial length of the piezoelectric stack and the application of a retracting force to the piston member, the risk of separation of the end member and the surface of the piston member is reduced. This permits opening movement of the valve needle of the fuel injector to be damped, thereby improving the control of valve needle movement and enabling relatively small quantities of fuel to be injected with improved accuracy.
Conveniently, the vent means may take the form of a passage, one end of the passage communicating with the volume and the other end of the passage communicating with the low pressure drain. The passage may be defined by a tubular member housed within the accumulator volume. Alternatively, the piezoelectric stack may be provided with a first bore, the passage being defined by a tubular member passing through the first bore provided in the stack or by the first bore itself.
The piston member conveniently includes a load transmitting member which defines the surface which engages the end member.
The fuel injector conveniently includes a valve needle which is slidable within a second bore and engageable with a seating to control fuel delivery through one or more outlet openings. The fuel injector may be of the inwardly opening type in which inward movement of the valve needle within the second bore causes fuel injection to be commenced.
The injector may further comprise damping means for damping movement of the piston member upon a reduction of the axial length of the piezoelectric stack. In this way, opening movement of the valve needle can be damped to improve control of valve needle movement.
Conveniently, the damping means may comprise a further chamber for fuel, the further chamber communicating with the accumulator volume by means of a restricted flow passage. Thus, in use, upon a reduction of the axial length of the piezoelectric stack and upward movement of the piston member, fuel within the accumulator volume is drawn through the restricted passage into the further chamber.
It is possible to damp opening movement of the valve needle as the risk of the end member and the surface associated with the piston member becoming separated is reduced. Thus, it is possible to provide an actuation stroke for a longer period of time in order to lift the valve needle away from its seating.
The further chamber may be provided with valve means operable in response to fuel pressure within the further chamber such that, upon movement of the piston member to reduce the volume of the further chamber, the valve means are caused to open to relieve fuel pressure within the further chamber.
This ensures closing movement of the valve needle is not hindered and fuel injection can be terminated rapidly.
The fuel injector may alternatively be of the outwardly opening type such that movement of the valve needle outwardly within the second bore causes fuel injection to be commenced.
The piston member may be provided with a piston bore which communicates with the volume, a first surface associated with the piston member being exposed to fuel pressure within the piston bore and a second surface of the piston member being exposed to fuel pressure within the control chamber.
Conveniently, the control chamber communicates with the accumulator volume such that, in use, during non-injecting stages of the injection cycle, fuel pressure within the control chamber is substantially the same as fuel pressure within the accumulator volume. Thus, leakage of fuel into the control chamber between fuel injections does not adversely affect fuel injector operation, as is the case in conventional fuel injectors of the outwardly opening type in which the control chamber is at reduced pressure during non-injecting stages of the injection cycle.